"I actually went back to being vegetarian when I became pregnant, just because I felt like I wanted that stuff," Portman told the radio Q100 "Bert Show" today. "I was listening to my body to have eggs and dairy."

Portman, who launched a vegan shoe line in 2008, said the strict diet that prohibits animal-derived products, from meat to honey, took its toll when she started craving sweets.

"If you're not eating eggs, then you can't have cookies or cake from regular bakeries, which can become a problem when that's all you want to eat," she said. "I actually wanted eggs at the beginning and then they grossed me out after a while."

Dr. Marjorie Greenfield, chief of obstetrics and gynecology at University Hospitals Case Medical Center in Cleveland, said it's common for women to make the switch during pregnancy.

"A lot of people do, I think, listen to their bodies and switch from being vegan to vegetarian when they're pregnant," Greenfield said. "Some people can just feel they're not getting enough and have the smarts to say, 'My body is telling me something and my baby is more important."

Greenfield said she's glad Portman is sending that message.

"In general, listening to your body is important," she said. "Not feeling good can be a sign there's something you should do differently."

And while a healthy vegan pregnancy is possible, it's tricky.

"I know there are people who do stay vegan," Portman told Q100, "but I think you have to just be careful, watch your iron levels and your B12 levels and supplement those if there are things you might be low in your diet."

Vegans often need to take supplements like iron and vitamin B12.

The biggest challenge for vegan moms-to-be is getting the right kind of protein and enough of it.

"In order to make whole proteins there are certain essential amino acids your body can't make. You have to combine your vegetable protein to make them," Greenfield said.

Gain Weight During Pregnancy

For babies to grow, mom has to gain weight too.

"If you're not gaining weight, that would certainly be a red flag," Greenfield said. "If you have a poor diet, the baby cannot grow well."

During pregnancy, the Centers for Disease Control and Preventions recommends six to 11 servings of grain products, three to five servings of vegetables, two to four servings of fruits, four to six servings of milk and milk products, and three to four servings of meat and protein foods every day.

"If you're not eating a standard diet, the take-home message is educate yourself about where the gaps might be and how you can fill them in a healthy way," Greenfield said.